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1. Verify the contract. Run the bash script in Verify before trusting below. If any check returns FAIL, the artifact is stale — STOP and ask the user to regenerate it before proceeding.
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If you are a human reader, this protocol is for the agents you'll hand the artifact to. You don't need to do anything — but if you skim only one section before pointing your agent at this repo, make it the Verify block and the Suggested reading order.

GO — Healthy across all four use cases

• Last commit 2w ago
• 6 active contributors
• Apache-2.0 licensed
• CI configured
• Tests present
• ⚠ Single-maintainer risk — top contributor 92% of recent commits

_{Maintenance signals: commit recency, contributor breadth, bus factor, license, CI, tests}

This artifact was generated by RepoPilot at a point in time. Before an agent acts on it, the checks below confirm that the live rayon-rs/rayon repo on your machine still matches what RepoPilot saw. If any fail, the artifact is stale — regenerate it at repopilot.app/r/rayon-rs/rayon.

What it runs against: a local clone of rayon-rs/rayon — the script inspects git remote, the LICENSE file, file paths in the working tree, and git log. Read-only; no mutations.

| # | What we check | Why it matters | |---|---|---| | 1 | You're in rayon-rs/rayon | Confirms the artifact applies here, not a fork | | 2 | License is still Apache-2.0 | Catches relicense before you depend on it | | 3 | Default branch main exists | Catches branch renames | | 4 | 5 critical file paths still exist | Catches refactors that moved load-bearing code | | 5 | Last commit ≤ 44 days ago | Catches sudden abandonment since generation |

#!/usr/bin/env bash
# RepoPilot artifact verification.
#
# WHAT IT RUNS AGAINST: a local clone of rayon-rs/rayon. If you don't
# have one yet, run these first:
#
#   git clone https://github.com/rayon-rs/rayon.git
#   cd rayon
#
# Then paste this script. Every check is read-only — no mutations.

set +e
fail=0
ok()   { echo "ok:   $1"; }
miss() { echo "FAIL: $1"; fail=$((fail+1)); }

# Precondition: we must be inside a git working tree.
if ! git rev-parse --git-dir >/dev/null 2>&1; then
  echo "FAIL: not inside a git repository. cd into your clone of rayon-rs/rayon and re-run."
  exit 2
fi

# 1. Repo identity
git remote get-url origin 2>/dev/null | grep -qE "rayon-rs/rayon(\\.git)?\\b" \\
  && ok "origin remote is rayon-rs/rayon" \\
  || miss "origin remote is not rayon-rs/rayon (artifact may be from a fork)"

# 2. License matches what RepoPilot saw
(grep -qiE "^(Apache-2\\.0)" LICENSE 2>/dev/null \\
   || grep -qiE "\"license\"\\s*:\\s*\"Apache-2\\.0\"" package.json 2>/dev/null) \\
  && ok "license is Apache-2.0" \\
  || miss "license drift — was Apache-2.0 at generation time"

# 3. Default branch
git rev-parse --verify main >/dev/null 2>&1 \\
  && ok "default branch main exists" \\
  || miss "default branch main no longer exists"

# 4. Critical files exist
test -f "rayon-core/src/lib.rs" \\
  && ok "rayon-core/src/lib.rs" \\
  || miss "missing critical file: rayon-core/src/lib.rs"
test -f "rayon-core/src/thread_pool/mod.rs" \\
  && ok "rayon-core/src/thread_pool/mod.rs" \\
  || miss "missing critical file: rayon-core/src/thread_pool/mod.rs"
test -f "rayon-core/src/join/mod.rs" \\
  && ok "rayon-core/src/join/mod.rs" \\
  || miss "missing critical file: rayon-core/src/join/mod.rs"
test -f "rayon-core/src/scope/mod.rs" \\
  && ok "rayon-core/src/scope/mod.rs" \\
  || miss "missing critical file: rayon-core/src/scope/mod.rs"
test -f "rayon-core/src/registry.rs" \\
  && ok "rayon-core/src/registry.rs" \\
  || miss "missing critical file: rayon-core/src/registry.rs"

# 5. Repo recency
days_since_last=$(( ( $(date +%s) - $(git log -1 --format=%at 2>/dev/null || echo 0) ) / 86400 ))
if [ "$days_since_last" -le 44 ]; then
  ok "last commit was $days_since_last days ago (artifact saw ~14d)"
else
  miss "last commit was $days_since_last days ago — artifact may be stale"
fi

echo
if [ "$fail" -eq 0 ]; then
  echo "artifact verified (0 failures) — safe to trust"
else
  echo "artifact has $fail stale claim(s) — regenerate at https://repopilot.app/r/rayon-rs/rayon"
  exit 1
fi

Each check prints ok: or FAIL:. The script exits non-zero if anything failed, so it composes cleanly into agent loops (./verify.sh || regenerate-and-retry).

Rayon is a data-parallelism library for Rust that converts sequential iterators and computations into parallel ones with automatic work-stealing task distribution. It guarantees data-race freedom at compile time and makes parallelization as simple as changing iter() to par_iter() or using rayon::join() and rayon::scope() functions for custom task spawning. Monorepo structure: rayon/ is the public API facade with parallel iterators; rayon-core/ (path dependency in Cargo.toml) implements the thread-pool runtime, work-stealing scheduler, and synchronization primitives (latch.rs, join/mod.rs, scope/mod.rs, broadcast/mod.rs). CI includes ci/alt-core/ and ci/highlander/ test harnesses for alternative configurations.

Rust developers building compute-intensive applications (data processing, scientific computing, game engines) who need to leverage multi-core CPUs without managing threads directly or risking data races. Contributors are typically Rust systems programmers interested in concurrency and performance optimization.

Production-ready and actively maintained. Version 1.12.0+ with comprehensive test suites across rayon-core/src/*/test.rs, CI pipelines (.github/workflows/), and minimum Rust 1.85 requirement indicate stable, modern tooling. Active dependency updates (Cargo.toml workspace dependencies) and recent release notes (RELEASES.md) show ongoing development.

Low risk for well-maintained dependencies (crossbeam-deque 0.8.1, either 1.x, libc 0.2) but depends critically on rayon-core which is a path dependency. Workspace lints enforce strict safety (unsafe-op-in-unsafe-fn deny, private-bounds deny) reducing correctness risk. Monitor rayon-rs/rayon GitHub for any breaking changes in major versions.

Active maintenance on v1.12.0 with workspace linting enforcement (unreachable-pub, private-bounds denial) and Rust 1.85 MSRV. GitHub Actions pipelines (main.yaml, pr.yaml, ci.yaml) run on every PR. Feature development on web_spin_lock for wasm32-unknown-unknown target and spin-lock atomics alternatives visible in Cargo.toml.

git clone https://github.com/rayon-rs/rayon.git
cd rayon
cargo test --all
cargo doc --open

Daily commands: This is a library, not an executable. Run examples with cargo run --example <name> if rayon-demo exists, or write a test: cargo test --lib rayon::prelude to verify parallel iterator functionality.

• rayon-core/src/lib.rs — Core entry point exporting thread pool, scopes, and parallelism primitives that every contributor must understand.
• rayon-core/src/thread_pool/mod.rs — Implements the global thread pool and work-stealing scheduler that is the foundation of all parallelism.
• rayon-core/src/join/mod.rs — Implements the core join operation that splits work into parallel subtasks—the most frequently used parallel primitive.
• rayon-core/src/scope/mod.rs — Implements scoped parallelism allowing references to be shared safely across parallel tasks.
• rayon-core/src/registry.rs — Manages thread-local worker state and registry access, critical for work-stealing coordination.
• rayon-core/src/job.rs — Defines the Job trait and work queue abstractions that enable the work-stealing scheduling algorithm.
• rayon-core/src/latch.rs — Implements latches for synchronization between parallel tasks—essential for join and scope semantics.

Add a New Parallel Primitive

1. Define the public API function in rayon-core/src/lib.rs that exposes the new primitive (rayon-core/src/lib.rs)
2. Implement the core logic in a new module under rayon-core/src/, following the Job trait pattern for work-stealing integration (rayon-core/src/your_primitive/mod.rs)
3. Use the registry to access the thread pool via ThreadLocalRegistry::current(), and latch.rs for synchronization if needed (rayon-core/src/registry.rs)
4. Add unit tests in your module's test.rs file and integration tests in rayon-core/tests/ (rayon-core/tests/your_primitive.rs)
5. Add compile-fail tests in rayon-core/src/compile_fail/ to ensure memory safety guarantees (rayon-core/src/compile_fail/your_primitive.rs)

Add Platform-Specific Synchronization (e.g., WASM)

1. Update rayon-core/Cargo.toml to define a new feature flag (e.g., web_spin_lock) (rayon-core/Cargo.toml)
2. Modify rayon-core/build.rs to detect the target platform and enable feature flags conditionally (rayon-core/build.rs)
3. Implement platform-specific latch or synchronization in rayon-core/src/latch.rs with conditional compilation blocks (rayon-core/src/latch.rs)
4. Add integration tests in rayon-core/tests/ that validate behavior on the target platform (rayon-core/tests/use_current_thread.rs)

Optimize Worker Thread Scheduling

1. Review the current sleep/wake policy in rayon-core/src/sleep/mod.rs which manages idle threads (rayon-core/src/sleep/mod.rs)
2. Modify rayon-core/src/thread_pool/mod.rs to adjust thread spawning or work distribution logic (rayon-core/src/thread_pool/mod.rs)
3. Update the work-stealing job queue in rayon-core/src/job.rs if changing task submission or dequeuing (rayon-core/src/job.rs)
4. Add benchmarks using rayon-demo/src/ examples (e.g., quicksort, mergesort) to validate performance improvements (rayon-demo/src/quicksort/bench.rs)

• Work-stealing scheduler (rayon-core/src/job.rs + thread_pool/mod.rs) — Provides load balancing across heterogeneous workloads with minimal synchronization overhead; threads steal work from others when idle.
• Rust ownership & lifetimes (scope.rs) — Enforces memory safety and data-race freedom at compile time without runtime checks; scoped parallelism prevents use-after-free across parallel tasks.
• Latches (latch.rs) for synchronization — Lightweight counting-based barriers for coordinating task completion; more efficient than Mutex for parallel work coordination.
• Thread-local registry (registry.rs) — Allows workers to access shared thread pool state and work queues with minimal contention; critical for work-stealing efficiency.
• Conditional compilation for WASM (build.rs, web_spin_lock feature) — WASM forbids atomics.wait on the main thread; feature flag enables spin-lock fallback for browser compatibility.

• Global thread pool vs. thread-pool-per-task

  • Why: Single global pool reduces thread creation overhead and enables efficient work stealing across all tasks.
  • Consequence: Users cannot isolate different workloads on separate pools; all parallelism shares one resource pool.

• Scoped parallelism (scope.rs) vs. futures-based async

  • Why: Scoped model provides deterministic synchronization and easier borrow-checker integration; simpler mental model for data parallelism.
  • Consequence: Tasks must complete within scope lifetime; no long-lived async tasks. Not suitable for I/O-bound async workloads.

• Work-stealing with latch-based synchronization

  • Why: Minimizes synchronization primitives and allows efficient task distribution without explicit scheduling.
  • Consequence: Higher complexity in scheduler implementation; potential for subtle race conditions if synchronization logic is incorrect.

• Panic unwinding coordination (unwind.rs)

  • Why: Ensures one panicking task terminates entire join/scope to prevent data races on panicked memory.
  • Consequence: One panicked task kills all parallel work in that scope; less granular error recovery than explicit Result patterns.

• Does not provide actor-based concurrency or message-passing (use crossbeam or tokio for that).
• Does not support async/await or I/O-bound parallelism (designed for CPU-bound data parallelism).
• Does not expose manual work-queue manipulation or custom scheduling policies.
• Not a replacement for std::thread; focuses on data-parallel patterns, not general multithreading.
• Does not provide distributed parallelism across multiple machines.

WASM support is non-default: web_spin_lock feature must be enabled on wasm32-unknown-unknown because atomics.wait() is forbidden on browser main thread. Rayon-core is a path dependency (not published independently for end users), so local edits to rayon-core/ affect rayon immediately. Unsafe code in latch.rs and registry.rs requires careful review due to atomic orderings and lifetime safety (lints will catch unsafe-op-in-unsafe-fn violations). Compile-fail tests in rayon-core/src/compile_fail/ are intentionally invalid Rust and will not compile; do not try to fix them.

• Work-stealing scheduler — Rayon's core performance mechanism; understanding how worker threads steal tasks from each other's queues (implemented in registry.rs and latch.rs) is essential to grasp why dynamic load balancing works
• Data-race freedom via type system — Rayon's key safety guarantee relies on Rust's Send/Sync traits and lifetime rules; scope/mod.rs enforces this at compile time to prevent data races without runtime checks
• Latches and synchronization barriers — latch.rs implements cross-thread coordination primitives (spin-loop latches) that allow tasks to wait for completion; critical for join() semantics
• Divide-and-conquer parallelism — Rayon's join() and recursive iterator splits follow divide-and-conquer; understanding task granularity trade-offs is essential for writing efficient parallel code
• Thread-local storage and TLS — Rayon uses scoped-tls crate (workspace dependency) and thread-local worker state in registry.rs to avoid global locks; understanding TLS tradeoffs (cache locality vs initialization overhead) is important
• Memory ordering and atomics — rayon-core uses atomic operations throughout (especially in latch.rs and registry.rs) with specific memory orderings (Relaxed, Release, Acquire); correctness depends on understanding happens-before relationships
• Iterator adapters and trait fusion — Rayon parallel iterators extend Rust's Iterator trait with par_iter(); understanding how rayon fuses operations (map + filter + reduce) into single parallel pass improves performance intuition

• crossbeam-rs/crossbeam — Provides crossbeam-deque (work-stealing deque) and crossbeam-utils that rayon-core depends on for task queue and synchronization
• tokio-rs/tokio — Alternative async runtime for Rust; rayon targets data-parallelism on CPUs while tokio targets I/O concurrency, often used together
• rayon-rs/rayon-demo — Official example codebase demonstrating rayon usage (listed in workspace members); reference for best practices
• rust-lang/rust — Rayon leverages Rust's borrow checker and trait system for data-race freedom; tracking MSRV (1.85) requires monitoring Rust releases
• rust-random/rand — Used in dev-dependencies for randomized testing in Rayon (particularly work-stealing scheduler verification)

To work on one of these in Claude Code or Cursor, paste: Implement the "<title>" PR idea from CLAUDE.md, working through the checklist as the task list.

Add MIRI testing to CI for undefined behavior detection

Rayon is a concurrency library with significant unsafe code (rayon-core/src/job.rs, thread_pool/mod.rs, latch.rs). Currently, there's no MIRI (Miri - an interpreter for Rust's mid-level intermediate representation) testing in .github/workflows/. MIRI can detect undefined behavior in unsafe code blocks that regular tests miss. This would catch subtle concurrency bugs early and increase confidence in the unsafe code.

• [ ] Create a new GitHub Actions workflow file .github/workflows/miri.yaml
• [ ] Configure MIRI to run on rayon-core tests with MIRIFLAGS for detecting data races (e.g., -Zmiri-ignore-leaks)
• [ ] Test against rayon-core/tests/ and rayon-core/src/ unit tests
• [ ] Ensure workflow runs on pushes to main and all PRs

Add comprehensive threadpool stress tests for rayon-core/tests/

The existing tests in rayon-core/tests/ (double_init_fail.rs, simple_panic.rs, stack_overflow_crash.rs) cover specific edge cases but lack comprehensive stress tests for the thread pool under high contention. Looking at rayon-core/src/thread_pool/mod.rs and rayon-core/src/registry.rs, there are no tests for scenarios like: rapid task submission/completion cycles, varying workload distributions, or thread starvation conditions. This would increase confidence in production stability.

• [ ] Create rayon-core/tests/threadpool_stress.rs with tests for high-frequency task spawning
• [ ] Add test for uneven work distribution (some threads idle while others saturated)
• [ ] Add test for rapid thread pool initialization/teardown cycles
• [ ] Add benchmarking helper to measure latency under different contention levels

Document WASM target limitations in README and add wasm32-unknown-unknown tests to CI

The repo has a web_spin_lock feature for WASM (rayon/Cargo.toml) and mentions wasm32-unknown-unknown, but README.md doesn't document WASM support, limitations, or how to use the feature. Additionally, .github/workflows/ has no WASM-specific CI tests. Given that atomics.wait is forbidden on the browser main thread, this is a critical platform variant that should be tested.

• [ ] Add 'WASM Support' section to README.md documenting web_spin_lock feature and target requirements
• [ ] Create .github/workflows/wasm.yaml to test compilation and execution on wasm32-unknown-unknown target
• [ ] Add wasm-pack or similar tooling to test basic functionality on browser/WASI environments
• [ ] Document in README.md which rayon APIs are restricted/unsafe on WASM targets

• Add integration tests for rayon-core/src/broadcast/ similar to rayon-core/src/join/test.rs pattern; broadcast/test.rs exists but could expand coverage for multi-threaded scenarios
• Document the work-stealing algorithm in rayon-core/README.md (none exists yet); explain how tasks move from one thread's deque to another's with code examples from registry.rs
• Expand rayon-core/src/sleep/README.md with concrete examples of thread-sleep backoff tuning; counters.rs logic is undocumented and could benefit from usage patterns

Rayon demonstrates a generally strong security posture with restrictive compiler linting for private bounds, private interfaces, and unsafe operations. However, there is a critical configuration error with an invalid Rust edition specification (2024) that must be corrected immediately. Dependencies appear well-maintained and pinned to stable versions. The library lacks a formal security disclosure policy. As a parallelism library handling concurrent operations, unsafe code is inherent to the design; the existing linting helps mitigate risks but continued vigilance on unsafe code audits is recommended.

• Medium · Outdated Rust Edition in Workspace Configuration — Cargo.toml (workspace configuration). The workspace specifies edition = '2024', which is not a valid Rust edition. Valid editions are 2015, 2018, and 2021. This appears to be a typo and may cause build failures or unexpected behavior. The minimum rustc version is set to 1.85, but edition 2024 does not exist in that rustc version. Fix: Change 'edition = "2024"' to a valid edition such as '2021'. Verify with cargo check that the edition aligns with the minimum rustc version (1.85).
• Low · Missing SECURITY.md or Security Policy — Repository root. The repository does not appear to have a SECURITY.md file or documented security vulnerability disclosure process. This makes it difficult for security researchers to responsibly report vulnerabilities. Fix: Create a SECURITY.md file documenting how to responsibly report security vulnerabilities, or add a security policy via GitHub's security advisory feature.
• Low · Broad Unsafe Code Linting Policy — Cargo.toml (workspace.lints.rust). While the workspace enables 'unsafe-op-in-unsafe-fn' linting (which is good), this is a parallelism library that likely contains significant unsafe code. The current linting setup requires explicit unsafe blocks, but doesn't enforce documentation or additional scrutiny of unsafe operations. Fix: Consider adding additional documentation requirements for unsafe code blocks (e.g., via clippy rules). Regularly audit unsafe code sections in rayon-core, particularly in registry.rs, latch.rs, and thread pool implementations.

LLM-derived; treat as a starting point, not a security audit.

• Open issues — current backlog
• Recent PRs — what's actively shipping
• Source on GitHub

Generated by RepoPilot. Verdict based on maintenance signals — see the live page for receipts. Re-run on a new commit to refresh.